Witnesses watched in horror as the North West Busways vehicle rolled down the middle of the town centre in Moville, Co Donegal, close to 8.30am.

It caused thousands of euro in damage to six cars including a brand-new €32,000 vehicle specially-adapted for a local wheelchair user.

Most of the damaged cars were parked and empty, but a childcare worker on her way to work suffered minor injuries as the bus smashed into the driver side of her vehicle.

There were no passengers on board the bus -- the 8.05am to Derry -- as they had already been asked to disembark after the driver encountered problems with the handbrake.

It is understood the driver went into a nearby shop, shortly before the empty bus took off down the street.

The bus travelled downhill for about 500 yards before crashing into an Alfa Romeo hatchback. This car was then pushed into the front of another.

Local hairdresser Bernard Doherty was shocked to discover that his wife Nora's car, which was parked in a disabled space outside their home, was among those damaged.

The Dohertys' car ended up on the footpath, blocking the doorway to a house.

"The car is a 2012 Ford Titanium operated by hand controls. It will be harder to replace. Nora was left disabled in a car accident many years ago, so this has been very distressing for her. We are glad nobody was killed," Mr Doherty said.

Staff at nearby Gillen's supermarket were among those who witnessed the events. They assisted a young woman driver who was injured and said to be traumatised by the incident.

"It's a miracle nobody was killed. We could hear all the loud bangs as the bus crashed into each of the different cars," supermarket owner, Paul Gillen, said.

One local garda dealt with the chaotic scene for an hour before back-up arrived as drivers tried to make their way to work and schools.

Bus company owner, Joe McGonagle, is trying to establish how it happened. "My understanding is that the driver applied the handbrake and let the passengers on. When he went to leave, the handbrake would not release, so he asked the passengers to get off."